Charge electrode

Abstract

There is provided a charge electrode for charging ink droplets for continuous ink jet printing. The charge electrode has a generally cylindrical main body defining a generally cylindrical passage for the ink droplets, said passage extending along a travel axis that represents, in use, the position of an ink jet in ingress to the electrode and a direction of travel of the ink droplets once these have detached from the ink jet. The charge electrode comprises two distinct axially disposed regions: a first region for charging the droplets as required; and, a second region for screening the charged droplets from any electric fields which could undesirably affect the trajectory of the ink jet and/or the droplets. The second region may fully surround at least a portion of the travel axis. The invention therefore provides improved screening for the ink jet and/or the droplets. This may in turn enable better control of the charge applied to the charged droplets, and/or of their travel trajectory.

The invention claimed is: 
     
       1. A charge electrode for continuous ink jet printers comprising:
 a passage, defined by the charge electrode, for forming and shielding charged ink droplets, said passage extending along a travel axis that represents, in use, a position of an ink jet that enters the charge electrode and from which ink droplets detach inside the charge electrode, and a direction of travel of the ink droplets before they exit the charge electrode, 
 wherein the charge electrode comprises directly abutting first and second axially sequentially disposed regions, wherein the first region is configured to induce a charge on ink droplets selected for printing by capacitive coupling, and the second region is configured to shield the charged ink droplets by surrounding at least a segment of said travel axis, wherein the charge electrode comprises an axial extension measuring 5 mm or longer. 
 
     
     
       2. The charge electrode of  claim 1  , wherein the charge electrode comprises a cylindrical body defining a body cylinder axis parallel with said travel axis. 
     
     
       3. The charge electrode of  claim 2 , wherein said cylindrical body comprises an outer stepped profile defining a cylinder length of increased diameter with respect to a rest of the cylindrical body, wherein said first region comprises said cylinder length and wherein said second region comprises at least a portion of the rest of the cylindrical body. 
     
     
       4. The charge electrode of  claim 2 , wherein the second region comprises an outlet aperture for the ink droplets; wherein the outlet aperture is located opposite an inlet aperture; wherein a size of the inlet aperture is less than that of the outlet aperture. 
     
     
       5. The charge electrode of  claim 2 , wherein the charae electrode further comprises a viewing aperture for monitoring formation of the ink droplets;
 wherein said viewing aperture is provided on a side of said cylindrical body. 
 
     
     
       6. The charge electrode of  claim 5 , wherein said viewing aperture comprises elongated shape that extends in the direction of travel and/or cylinder axes. 
     
     
       7. The charge electrode of  claim 5 , wherein the first and the second axially sequentially disposed regions are located one on either side of said viewing aperture. 
     
     
       8. The charge electrode of  claim 5 , wherein the first and second axially sequentially disposed regions define opposite ends of the charge electrode. 
     
     
       9. The charge electrode of  claim 5 , wherein the first and second axially sequentially disposed regions adjoin each other. 
     
     
       10. The charge electrode of  claim 1 , wherein said passage is cylindrical and defines a passage cylinder axis parallel with said travel axis. 
     
     
       11. The charge electrode of  claim 10 , wherein the charge electrode is a coaxial charge electrode and a cylinder axis of a cylindrical body of the charge electrode and/or the passage cylinder axis overlap with said travel axis, or one with the another. 
     
     
       12. The charge electrode of  claim 1 , wherein said first region comprises an inlet aperture for the ink jet. 
     
     
       13. The charge electrode of  claim 1 , wherein said passage comprises a step defining a stepped profile for the passage; wherein said step separates and/or identifies said first and second axially sequentially disposed regions;
 wherein the passage is narrower in correspondence of the first region than the second region. 
 
     
     
       14. The charge electrode of  claim 1 , wherein the charge electrode comprises a mounting and/or orientation feature for mounting and/or registering in place, in use, the charge electrode on a printhead or a printhead deck. 
     
     
       15. The charge electrode of  claim 14 , wherein the mounting and/or orientation feature is in the form of a flat provided on the charge electrode; wherein said flat is provided on said second region. 
     
     
       16. The charge electrode of  claim 1 , wherein a length of the charge electrode is greater than 6 mm; or, greater than 7 mm; or, greater than 8 mm; or, greater than 9 mm; or, greater than 10 mm; wherein said lengths relate to said second region. 
     
     
       17. The charge electrode of  claim 1 , wherein the a length of said segment surrounded by said second region is 2 mm or longer. 
     
     
       18. The charge electrode of  claim 1 , wherein the charge electrode comprises an axially elongated shape. 
     
     
       19. The charge electrode of  claim 1 , wherein the first region is structured and/or configured to be different compared to the second region. 
     
     
       20. The charge electrode of  claim 1 , wherein the first region is better structured and/or configured to induce said charge on the charged ink droplets than the second region. 
     
     
       21. The charge electrode of  claim 20 , wherein the second region is better structured and/or configured to shield said charged ink droplets than the first region. 
     
     
       22. The charge electrode of  claim 1 , wherein the first region is configured to induce said charge on said ink droplets selected for printing for the ink droplets selected for printing to be deflected by one or more deflection plates for printing onto a moving substrate at predetermined positions. 
     
     
       23. Apparatus for a printhead comprising a charge electrode according to  claim 1 . 
     
     
       24. A printhead comprising:
 a nozzle for generating a continuous ink jet and ink droplets detached therefrom; 
 a charge electrode as claimed in  claim 1  or apparatus according to  claim 23 ; 
 one or more deflection plates for deflecting the charged ink droplets charged by said charge electrode for printing on a moving substrate associated with the printhead; and 
 a gutter for recirculating any uncharged ink droplets. 
 
     
     
       25. The printhead of  claim 24 , wherein the nozzle is configured to emit the continuous ink jet at a velocity V, the printhead is arranged to form the ink droplets at a frequency F, and the ink droplets are broken off from the continuous ink jet at a droplet pitch DP=V/F; and wherein a length of the charge electrode is greater than 15 droplet pitches. 
     
     
       26. The printhead of  claim 25 , wherein the length of the charge electrode is less than 70 droplet pitches. 
     
     
       27. An apparatus comprising the printhead as claimed in  claim 24 . 
     
     
       28. The apparatus of  claim 27 , wherein said apparatus is a continuous ink jet printer. 
     
     
       29. A method of printing, the method comprising:
 generating a continuous ink jet; 
 forming a plurality of ink droplets from said continuous ink jet; 
 using a charge electrode as claimed in  claim 1 , selectively charging the ink droplets for printing on a moving substrate; 
 deflecting any charged ink droplets to print them on the moving substrate at predetermined positions.